Conventional techniques will be explained below. For example, in a mobile station accommodated in a mobile communication system that employs a CDMA system, a reception timing of a receiving channel corresponding to a reception signal is detected by executing an inverse diffusion processing and a demodulation processing to pilot signals received in a predetermined period. Usually, in a mobile communication environment, reception signals are detected as a plurality of reception timing candidates in a RAKE path detector. In this case, paths detected as candidates have mutually different correlation values and reception timings.
At a mobile station, a main reception timing is determined from among the plurality of reception timing candidates. Specifically, at the mobile station, a reception reference timing held in advance is compared sequentially with the latest reception timing candidates. Then, the reception reference timing, that is, a clock to be managed inside, is corrected so that the reception reference timing coincides with an optimum reception timing that has been determined based on a result of the comparison.
In this way, at the mobile station, the optimum reception reference timing is obtained by absorbing a clock deviation between the mobile station and a base station, and a clock deviation due to a variation in a transmission distance, based on a clock correction.
As a concrete example of a timing correcting device for obtaining the optimum reception reference timing, there is a device disclosed in Japanese Patent Application Laid-open (JP-A) No. 11-261410, for example. FIG. 4 is a diagram showing a structure of a conventional timing correcting device described in the above publication. In FIG. 4, 101 denotes a reception timing detecting circuit, 102 denotes a comparator circuit, 103 denotes a reference timing generating circuit, 104 denotes a clock generating circuit, 105 denotes a timing correction control circuit, and 106 denotes a correction speed control circuit.
The operation of the timing correcting device will be explained. To begin with, the reception timing detecting circuit 101 generates a reception timing signal S(2) based on a pilot signal S(1) included in a reception signal, and outputs this to the comparator circuit 102.
On the other hand, the reference timing generating circuit 103 generates a reference timing signal S(4) of a period that is substantially the same as the period of the reception timing signal S(2), based on a clock signal S(3) received from the clock generating circuit 104, and outputs this to the comparator circuit 102.
The comparator circuit 102 compares the reception timing signal S(2) with the reference timing signal S(4), and outputs a result of this comparison to the timing correction control circuit 105 as a comparison result signal S(5).
When the comparator circuit 102 has detected a deviation between the reception timing signal S(2) and the reference timing signal S(4), the reference timing generating circuit 103 corrects the reference timing signal S(4) so that this coincides with the reception timing signal S(2).
However, in general, a reception wave in a radio communication is transmitted as a “multi-path” consisting of a direct wave that arrives directly and a plurality of reflection waves that arrive after reflecting from buildings or the like. An amount of delay in a reflection wave from a direct wave is not constant, and this amount of delay changes dynamically as it is controlled by the surrounding buildings and the topography. Further, at the mobile station, a reception timing of a reception wave changes every moment due to a change in the propagation route according to a move.
Thus, in the mobile communication environment, a direct wave is not always received, and sometimes the reception timing is detected from the reflection wave, and a reception reference timing is corrected by mistake in such a manner that it coincides with the reception timing of the reflection wave. Moreover, sometimes, at the next timing, a reception timing is detected from a direct wave, and a reception reference timing is corrected such that it coincides with the reception timing of the direct wave.
As explained above, according to the conventional clock correcting device described in the above-mentioned reference, there has been a problem that, when there is a large propagation path difference between a direct wave and a reflection wave, a reception reference timing is corrected frequently, and a switching of a received RAKE path occurs frequently, with a result that the load amount of correcting a reception reference timing increases. Further, according to the conventional clock correcting device, there has also been a problem that the frequency of updating a path allocation to a finger increases due to an erroneous tracking of a reception reference timing.
The present invention has been made in the light of the above problems. It is an object of this invention to provide a method of and a device for correcting a timing that is capable of determining a reception reference timing efficiently and in optimum, by realizing a reduction in the load amount of correcting a reception reference timing and a reduction in the frequency of updating a path allocation.